This invention relates generally to electronic automatic gain control techniques, and, more specifically, to the use of such techniques in the measurement of a level of infra-red radiation emitted by an object surface whose temperature is being measured.
There are many applications where the intensity of the emission from an object surface of infra-red radiation is detected by a photodetector and measured in order to determine the temperature or some other parameter of the object surface. One such application utilizes a small black-body cavity formed on an end of an optical fiber as a temperature sensor, emissions from the cavity being transmitted to a photodetector at the other end of the optical fiber. The temperature of the environment in which the black-body sensor is positioned is thus measured. An example of such a system is disclosed in U.S. Pat. No. 4,750,139. Since the intensity level of the infra-red emissions varies considerable over a temperature range of interest, as much as an order of magnitude or more, an automatic gain control circuit is used in a front end of a measuring instrument in order to maintain a temperature proportional analog signal at a level within a desired input range of an analog-to-digital converter (DAC) or other signal processing device. The automatic gain control is typically formed of an analog circuit including at least one amplifier whose gain is set through a switched resistor network.
Pyrometers have also been used for a long period of time for measuring the temperature of a surface without having any contact with that surface. Infra-red emissions from the surface are remotely imaged onto a photodetector and processed in order to determine the surface temperature. More recently, optical fibers or other light pipes are used as part of the infra-red radiation transmission and imaging system.
One important current application of such non-contact temperature measurement is in the field of the formation of integrated circuits. Several steps of forming an integrated circuit on a semi-conductor wafer are performed within a reaction chamber. The temperature of the wafers is rapidly cycled through a predetermined temperature profile in a class of processes referred to as rapid thermal processing (RTP). The necessity to follow predetermined temperature profiles during the time of processing is a result of the very small dimensions of the various regions and components of the integrated circuits so formed. Accurate, real time measurement of the wafer's temperature is thus quite important to the success of RTP.
Almost all semi-conductor processing is performed on silicon wafers. Silicon has the characteristic of being transparent to radiation having a wavelength in excess of about one micron. Therefore, it is usually desired to measure the level of infra-red emissions from a silicon wafer at a wavelength of less than one micron. Heretofore, such a technique has been useable to measure temperatures only down to about 500.degree. C. This is because the infra-red emissions from the wafer surface, as with any such surface, drop to a level that is so low at low temperatures that current techniques cannot accurately measure the emissions. The signal-to-noise ratio becomes quite low, making accurate measurements very difficult.
Therefore, it is a principle object of the present invention to provide individual techniques and a combination of techniques which allow making such temperature measurements with improved accuracy.
It is a specific object of the present invention to provide individual techniques and a combination of techniques for allowing the measurement of semi-conductor wafer temperatures by monitoring emission wavelengths less than one micron of temperatures in wider ranges than now possible to be made with a high degree of accuracy.
It is a more general object of the present invention to provide an improved automatic gain control technique for use in the front end of instruments used to make such measurements.
It is an even more general object of the present invention to provide a technique for digitizing analog signals which can vary over a range that is much larger than a preferred input range to an analog-to-converter or other signal processing device.
It is an additional object of the present invention to provide an improved physical arrangement of optical and electronic components for detecting the level of radiation emissions.